animation: also change stop value when updating a fade target

animation: also change stop value when updating a fade target

If we're fading a view with weston_fade_run() and half way through we
want to unfade it we can use weston_fade_update() to set a new alpha
target. When the animation finishes, reset_alpha() is called which
ensures the alpha value of the view is exactly as requested.

Although weston_fade_update() was updating the spring target so the
animation would look fine, it would then reset to the alpha target
given in the first call to weston_fade_run().